DFB lasers do not utilise the cavity mirrors of conventional Fabry-Perot systems, but rather rely on backward Bragg scattering from periodic perturbations of the refractive index and/or gain of the laser medium to provide the optical feedback necessary for laser operation. DFB structures have the advantage of providing better frequency stability of the mode of oscillation than Fabry-Perot structures.
In semiconductor DFB lasers, the periodic perturbations are generally provided by means of a grating formed, usually, in a semiconductor layer adjacent the device's active layer, the teeth and grooves of the grating extending orthogonally to the device's optical axis. Typically the grating extends throughout the entire length of the laser, but in some devices the grating is shorter than the device, the grating ends being remote from the device ends.
The wavelength sensitivity of the Bragg effect results in DFB lasers exhibiting a high degree of spectral selection. The narrow linewidth of DFB lasers means that they are very attractive for use in optical communications systems, not least because of the effective increase in bandwidth that can be achieved as a result of the reduced dispersion consequent on the use of a narrower linewidth source. Unfortunately, however, the optical output of DFB lasers, while of narrow linewidth, is not absolutely monochromatic: generally two and sometimes three longitudinal modes will be supported simultaneously, the dominant mode having the highest intensity output. If the power difference between the dominant and subordinate modes is great enough, the laser's output can be considered for some purposes to be single mode. The problem lies in ensuring a sufficiently great power difference. Typically, the aim is to achieve side to main mode power ratios of--40 dB or better under the most severe operating conditions--in particular, under direct modulation.
As reported by Haus and Shank in IEEE Journal of Quantum Electronics, Vol. QE12, No. 9, pp 532-539, 1976, the mode spectrum of the original DFB laser as analyzed by Kogelnik and Shank (J. Appl. Phys., Vol. 43, pp 2327-2335, 1972) consisted of modes of equal threshold on either side of a gap at a `centre` frequency. Haus and Shank note that this threshold degeneracy is a disadvantage in practical applications where single-mode operation at a predictable frequency is desired, and they show that antisymmetric tapering of the coupling coefficient of the period of the structure may be utilized to remove the threshold degeneracy. Haus and Shank established that all structures with an antisymmetric taper of K (the feedback parameter of Kogelnik and Shank) support a mode at the centre frequency of the local stopbands. This mode has a particularly low threshold when used in a laser copy.
Haus and Shank found that DFB lasers with a stepped-K structure, that is one with a phase shift between a first section of grating and a second section of grating, had no threshold degeneracy and had much better threshold discrimination between the fundamental mode and the first higher order mode. They also found the frequency separation between the dominant mode and the first order mode to be much greater for the stepped structure than for the uniform structure.
As a result of the work by Haus and Shank, DFB lasers are now made with phase-shifted gratings.